1. Field of the Invention
The present invention relates to vehicle air conditioning apparatus which is capable of preventing the freezing of a heat exchanger and which functions as an evaporator in the air conditioner's cooling and dehumidifying modes.
2. Related Art
Conventionally, in an air conditioner for a vehicle, in order to prevent condensed water on the surface of a heat exchanger while functioning as an evaporator in the air conditioner's cooling and dehumidifying modes, from freezing and building up ice deposits, a lower operational temperature of the heat exchanger is set beforehand to prevent freezing. Any decrease in effectiveness of the heat exchanger and damage to the system's compressor due to liquid being fed to the compressor (i.e., a liquid back phenomenon) are prevented by controlling the temperature of the heat exchanger so that it is not lower than the lower operational temperature.
However, in a vehicle air conditioner implementing the above freeze prevention function, when the system is in a low temperature environment (such as in the winter) and is operating in a dehumidifying mode to avoid fogging effects, the freeze prevention function may terminate the heat exchanger operation more quickly than is desirable (even before the dehumidifying mode is started), thereby causing a problem.
In such a situation, it is advantageous to supply low-humidity outside air into the passenger compartment or to operate the system in a heating mode and then perform the dehumidifying operation. However, if the outside air is dirty (e.g., when the vehicle is travelling through a tunnel or when it is travelling on an unpaved road), such outside air preferably is not supplied to the passenger compartment. So, the first above-mentioned technique in which outside air is supplied during the dehumidifying operation does not produce satisfactory results. When the system is operated in a heating mode and the dehumidifying operation is then performed as in the second above-mentioned technique, it is impossible to perform a dehumidifying operation until the engine has warmed-up. Therefore, the dehumidifying operation cannot be performed during engine start-up in a cold environment.
As discussed above, when the temperature of an evaporator in a conventional air conditioning system is equal to or lower than a set temperature during a refrigeration cycle, operation of the system's compressor is stopped to prevent ice formation on the evaporator. In order to perform a defogging operation reliably, Japanese Patent Laid-Open Publication No. Sho 63-71414 discloses that while the defogging operation is performed, operation of the compressor is continued for a predetermined time period measured by a timer even when the temperature of the evaporator is equal to or lower than the set temperature, thereby performing a defogging operation in spite of the low outside air temperature.
However, the amount of condensed water collecting on the surface of the evaporator depends on the temperature of the intake air which exchanges heat with the evaporator. So, in the above system in which the operation of the compressor is continued for a predetermined time period independently of the evaporator temperature, when the temperature of the intake air is high, the amount of condensed water which collects on the surface of the evaporator during freezing conditions may become extraordinarily large. In such a situation, it is impossible to avoid deterioration of the evaporator's heat exchanging ability, and damage to the compressor may result from liquid back effects. Further, according to the above publication, the compressor is stopped after a predetermined time period measured by the timer has passed. However, when the temperature of the intake air is low (e.g., when the outside temperature is low), the condensed water which has frozen on the evaporator may remain if the compressor is simply stopped. Thus, when the compressor resumes operation, the likelihood that the heat exchanging ability of the evaporator will be adversely affected may be increased.